The present invention relates to a method and to devices for transmitting and receiving data in a code division multiple access telecommunication system.
A telecommunication system is a system, in which data are communicated between one or more base stations and one or more mobile stations. Thereby, the communication area is divided in cells, in which one base station communicates with one or more mobile stations. Multiple access systems are used to support the simultaneous access of a plurality of mobile stations to one base station within the limited resources of the transmission system. Several multiple access systems are known, e.g. frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA). Additional to these basic types of multiple access systems, combinations of these different systems are possible and in practical use. The GSM-System for example uses a combination of FDMA and TDMA.
The present invention particularly relates to the transmission and reception of random access data in a code division multiple access system. Random access data are transmitted in the so-called random access slot (RACH) from a mobile station to build up a connection or to transmit data. The random access data from the mobile station contain a request, if the base station has sufficient resources available to build up the required connection or to transfer user data.
The random access slot comprises or consists of succeedingly or periodically provided random access time windows, in which several random access slots are available. The different random access slots are randomly chosen by a mobile station for the transmission of random access data. In a currently proposed wide band direct sequence CDMA (WCDMA) system, the random access time windows are based upon an initial preamble scrambling code, which differentiates between one cell and another cell. Thereby, these codes need to be planned to ensure that neighboring cells do not use the same code. Therefore, within the preamble part of each random access slot burst, is provided the preamble signature, which is one of 16 separate codes available for use within that cell. These 16 codes can be seen as separate slots. One of these codes is chosen randomly by the mobile station for the transmission of random access data. Beforehand, the base station broadcasts, which codes are available in each cell over the broadcast control channel (BCCH). In addition, within one time frame (10 ms) are provided 8 time offsets, each of 1,25 ms, allowing a further 8 variations. In other words, in each time frame a random access time window is provided, which comprises a plurality of random access slots for transmitting random access data from one or more mobile stations to the base station. The random access time window thereby extends over the time frame of 10 ms, so that 128 different random access slots (16 separate preamble codes and 8 time offsets) are provided within one random access time window.
A collision, i.e. a situation, where messages from two or more mobile stations collide and are lost, only occurs, when both the preamble code and the time offset are chosen in the same random access time window. In practice, it is expected that only about 30% of the theoretical maximum of 128 accesses per 10 ms will be possible.
In a situation, where a number of packet data users are operating in a bursty traffic situation, this maximum could be quickly reached. In such a situation the access to the network will either become slower or not be possible at all. This is due to congestion caused by the build up of first time requests and the retransmissions made necessary by previous collisions. Since the access to the random access slots is only contention based as shown in FIG. 1, a guaranteed upper limit on the amount of time needed to access the system even after an initial burst is not ensured. For packet data applications, which demand a constant periodic delivery of data, ready access is critical.
WO 98/24250 discloses a TDMA system, which allocates a fixed random access slot constantly to a particular mobile station.
The object of the present invention is thus to provide a method and devices for transmitting and receiving data in a code division multiple access telecommunication system, in which a random access time window comprising a plurality of random access slots for transmitting random access data is provided, whereby the random access time window is partioned on the basis of a partition point in the first section comprising contention based random access slots and a second section comprising reservation based random access slots, and in which an optimal setting of the partition point is ensured, so that the random access time window resources are fully utilized. The importance of an effective random access slot utilization arises particularly from increased load from packet data capacity requests and the transfer of small amounts of user data within random access slot burst.
This object is achieved by a method for transmitting and receiving data in a code division multiple access telecommunication system, comprising the steps of providing a random access time window comprising a plurality of random access slots for transmitting random access data from at least one communication device to a second communication device, determining a partition point defining a number of contention based random access slots and a number of reservation based random access slots in the random access time window on the basis of statistical information generated in at least one first communication device, and partitioning the random access time window in a first and a second section on the basis of said partition point, whereby the first section contains contention based random access slots and the second section contains reservation based random access slots. The first communication devices can thereby be mobile stations and the second communication device can thereby be a base station of the telecommunication system. Reservation based random access slots are slots which cannot be accessed on a contention basis, but only if they had been reserved before.
The above object is further achieved by a device for transmitting and receiving data in a code division multiple access telecommunication system, in which a random access time window comprising a plurality of random access slots for transmitting random access data is provided, the random access time window being partitioned in a first and a second section, whereby the first section contains contention based random access slots and the second section contains reservation based random access slots, with means for randomly choosing a random access slot from said first section, means for transmitting random access data in said chosen random access slot, and means for generating statistical information on contention based random access attempts. This device for transmitting and receiving data is e.g. a mobile station of the telecommunication system.
The above object is further achieved by a device for transmitting and receiving data in a code division multiple access telecommunication system, in which a random access time window comprising a plurality of random access slots for transmitting random access data is provided, with means for determining a partition point defining a number of contention based random access slots and a number of reservation based random access slots and a random access time window on the basis of received statistical information, means for partitioning the random access time window in a first and a second section on the basis of said partition point, whereby the first section contains contention based random access slots and the second section contains reservation based random access slots, and means for broadcasting information on the partition point of said random access time window. This device for transmitting and receiving data can for example be a base station of the telecommunication system.
The partitioning of the random access time window into the first section containing contention based random access slots and a second section containing reservation based random access slots allows to reduce congestion in the random access slots and to more efficiently utilize the resources of the random access time window. The determination of the partition point on the basis of statistical information generated e.g. in a mobile station allows an optimal setting of the partition point, so that random access time window resources can be fully utilized and initial access requests or random access slot reservations can be optimized. The present invention thus provides an automated mechanism for the determination of the partition point, so that the random access time window can be flexibly modified from one moment to the next moment to adapt the random access time window to dynamically changing environments and requirements.
Advantageously, the statistical information is information on contention based random access attempts in said at least one first communication device, which e.g. can be a mobile station. Before the transmission of random access data in a contention based random access slot, a mobile station randomly chooses a random access slot from the first section of the random access time window. Then, the mobile station transmits random access data in this chosen random access slot. At this time, the mobile station does not know if the transmission of the random access data in the chosen random access slot was successful. Only upon receiving of a corresponding notification from the respective base station, the mobile station is informed, if the random access data transmission attempt was successful. The first communication device, which e.g. can be a mobile station, thus comprises a means for generating statistical information on these contention based random access attempts. Advantageously, said means for generating statistical information determines the number of failed contention based random access attempts within a predetermined period of time. In this case, said means for generating statistical information can accumulate the number of failed contention based random access attempts and provide the corresponding second communication device, which e.g. can be a base station, with corresponding statistical information. The corresponding second communication device, which can e.g. be a base station, receives the corresponding statistical information from one or more first communication devices, which can be mobile stations, and sets the partition point of the random access time window correspondingly. Thereby, information on the partition point is, e.g. periodically, broadcast to a mobile station.
Said device for transmitting and receiving data according to the present invention, which can be a base station, comprises a means for determining said partition point on the basis of the number of failed contention based random access attempts in one or more further devices, which can e.g. be mobile stations, within a predetermined period of time. Said means for determining the partition point advantageously increases the number of contention based random access slots in said random access time window in case that the number of failed contention based random access attempts reaches a predetermined contention threshold value. Thereby, said means for determining the partition point advantageously decreases the number of contention based random access slots in said random access time window in case that the number of failed contention based random access attempts falls under a predetermined reservation threshold value. Preferably, said reservation threshold value is smaller than said contention threshold value, whereby a buffer space is provided in between the reservation and the contention threshold value. Within the buffer space, the number of failed contention based random access attempts can vary without a subsequent changing of the partition point, so that small oscillations of the partition point are avoided and the amount of the required signaling is decreased.
In an alternative embodiment of the device for transmitting and receiving data according to the present invention, which can be a base station, said determining means determines the partition based on the basis of the number of failed contention based random access attempts of one or more further devices, which can e.g. be mobile stations, within a predetermined period of time, whereby the partition point is calculated arithmetically. In this embodiment, no contention or reservation threshold values are set, but the partition point is varied according to predetermined mathematical criteria.
Each random access slot in said random access time window can be defined by a time offset value and a preamble code. Further, said second communication device (which e.g. can be a base station) periodically transmits partitioning information defining the partition point of the random access time window to at least one first communication device, which for example can be a mobile station. The first communication device, which for example can be a mobile station, randomly chooses one of said random access slots from said first section of said random access time window for transmitting random access data to the second communication device, which for example can be a base station.
Advantageously, said random access data transmitted in a random access slot of said first section comprises reservation data for reserving at least one random access slot of said second section in at least one succeeding random access time window. Thereby, ready access for packet data applications is ensured, which demand a constant delivery of data. In this case, said reservation data can comprise data on the time duration required for the total number of reserved random access slots to indicate the end of the reservation. Thereby, the reservation of random access slots in the second section of succeeding random access time windows ends automatically upon expiring of the pre-set time period. Said reservation data can further comprise information on a number of random access slots to be reserved in the first succeeding random access time window and information on a continues reduction of said number in the following random access time windows. In this case, the number of reserved random access slots in the second sections of succeeding random access time windows can be decreased gradually until the end of the reservation term.
Said first communication device, which can for example be a mobile station, can transmit reservation termination data in a first or second section of a random access time window for indicating the end of the reservation of the required random access slot. This might be necessary in cases, in which already reserved random access slots in the second sections of succeeding random access time windows are not necessary and have to be released for usage by other users. Alternatively, said second communication device, which can for example be a mobile station, terminates the reservation upon determination of a non-usage of reserved random access slots. The second communication device can thereby automatically terminate the reservation if it is determined, that reserved random access slots are not used. In this case, the second communication device can transmit information on the termination of the reservation to the corresponding first communication device to inform that earlier reserved random access slots in second sections of succeeding random access time windows are not reserved anymore.
The device for transmitting and receiving data according to the present invention, which might be a mobile station, can further comprise means for receiving periodically transmitted partitioning information defining the partition point of said random access time window.
The device for transmitting and receiving data according to the present invention, which might be a base station, might further comprise means for receiving random access data in one of said random access slots from said first section of said random access time window. Said means for partitioning the random access time window advantageously sets the number of random access slots in said first and second section respectively, variably depending on the partition point, whereby said means for transmitting partition point information periodically transmit said partitioning information. Said device for transmitting and receiving data, which might be a base station, can further comprise means for determining a non-usage of reserved random access slots, whereby the reservation is terminated upon a positive result of said determination. Said means for transmitting information can thereby transmit information on the termination of the reservation upon a positive result of said determination.
In the method and devices for transmitting and receiving data according to the present invention, the random access time window is partitioned into a first and a second section, whereby the first section contains contention based random access slots, and the second section contains reservation based random access slots. The reservation based random access slots can be dedicated to a communication device, e.g. a mobile station, for a limited time. A request for guaranteed reservation of reservation based random access slots is made through an initial contention based random access slot. Thereby, the network or the base station may also assign a reserved random access slot. These reserved random access slots are then used for uplink data transfer between a mobile station and a base station or requests for a channel to transmit user data. Since the partitioning of the random access time window can be set variable depending on system requirements, the random access slot resources in the random access time window can be used more efficiently, e.g. by minimizing contention based access, since band-width is not used for retransmissions when bursts collide. Furthermore, a breakdown can be avoided, which can occur, when the number of retries and subsequent retransmissions increase the number of collisions until deadlock occurs. The telecommunication network or the base station can dynamically control the partitioning of the random access resources in the random access time window into contention and reservation based channels. Thus, the exact partition can be tailored to the particular requirements from moment to moment. This partition point information is broadcast to the mobile stations upon the broadcast control channel (BCCH). The present invention thereby particularly enables an optimal utilization of the random access time window resources and a faster access to the communication network particular in cases where a number of packet data users are operating in bursty traffic situations. Further, congestion caused by the build up of first time requests and retransmissions made necessary by previous collisions can be avoided.